Art of cracking hydrocarbons



Aug. 27, 1940.

O. G` KAASA ART OF CRACKING HYDROCARBONS Filed Nov. 14, 1952 QSC/m N ATTORNEYS taining substantial proportions of gasoline, 'at

Patented Aug. 27, 1940.

r ART 0F CRACKING HYDRDCARBONS Orin G. Kaasa, Munster, Ind., assigner to Sinclair Refining Company, New York, N. Y., a

corporation of Maine Application November 14, 1932, Serial No. 642,496 2 claims. j (c1. iss-61) `My invention relates to improvements in the cracking of petroleum stocks containing a substantial proportion of gasoline at 'high cracking temperatures for-the production of gasoline fractions of improved anti-knock value. In refer.

ring to stocks containing a substantial proportion of gasoline, I refer to stocks of which more than 40% boils off up to 420 F. at atmospheric pressure and intend to include stocks of which as much as or `more boils off at 420 F. under `atmospheric pressure. In referring to lhigh `cracking temperatures, I refer to cracking temperatures upwards of `890" F. Operations of this general character are sometimes designated reforming operations. g

By cracking gasoline fractions, or stocks conhighcracking temperatures, the gasoline content of the stock as well `as higher boiling components are cracked with consequent formation, under appropriate conditions which are well known, of gasoline fractions of anti-knock value improved with respect to that of the original gasoline content of the stock. The cracking temperatures useful for this purpose are so high that, particularlylwith respect to such stocks, the stock is usually completely vaporized except for particles of 4gum constituents, tar constituents or very high boiling lconstituents of the stock 4which remain unvaporiz'ed as a fog or mist in the operation. Under .appropriate conditions, such normally small proportions `of unvaporized material can be maintained in suspension in la rapidly flowing stream of vapors `and thus Acarried through and discharged from the heating involved in such p operations, but prior to that `point in the operation at which the rchange in phase from the liquid state in which the stock is supplied to the heating to the `vapor state in which it is discharged from the heating ceases any segregation of such gum constituents, tar vconstituents or very high boiling constituents tends to involve overheating `of the conduit, for example, in which the heating is effected and any overheating of this portion of the conduit tends to involve such `segregation and these two tendencies 'become mutually cumulative. Even though the hazards of apparatus failure be ignored, these tendencies involve limitations of the capacity of the apparatus in which such operations are carried out. If the effect of these tendencies is ignored, except `as it requires termination of the operation for cleaning of the apparatus, the proportion of time over which the apparatus can be usefully operated is decreased and .if the apparatus is` operated at a lower heating rate either to reduce `the rate of segregation of such constituents or to reduce the effect of such-'segregation as occurs, the l capacity of `the apparatus is correspondingly decreased. f

I vhave found, however, that Aby limitingl the rate of heat transfer to the stock from a point in advance of that at which substantial vapori- Zation begins to the point at which the phase change substantially ceases as compared to the rate of heat transfer to the stock beyond the `point at which the phase change substantially ceases, and more particularly immediately beyond this point, these tendencies can be eliminated, at least in a substantial measure, without substantial prejudice to the capacity ofthe apparatus in which `the operation is conducted.

According to my invention, the stock to be cracked :is supplied to a heating conduit substantially in liquid phase, it is heated to Va highn cracking temperature therein while flowing therethrough asa stream, and it is discharged vtherefrom substantially in `vapor phase, and a lower rate of heat transfer per unit of area of theI heating conduit is maintained from a point in advance of that at which substantial vapori- Zation begins to the point at which the phase change, from the liquid phase in which Vit is supplied to the heating conduit to the state in which it is discharged therefrom, substantially ceases than immediately beyond the ylast mentioned point.

My invention is of special value in conjunction withoperations, or that `part of operations, in which the heat supplied to the oil during the heating is transferred principally as radiant energy.` Realization of a full measure of capacity available in apparatus of this type usually involves very high rates of' heat transfer. In operations conducted in such apparatus, the tendencie-s toward overheating and segregation to which I have referred are particularly objectionable and the elimination of these tendencies affords an unusual measure of advantage. In carrying out my invention in such apparatus, the heating conduit is shielded from the source` of radiant energy from a point in advance of `that at which substantial vaporizaticn begins to the point at which the phase change substantially ceases, a differential between the rates of heat transfer in advance of the last-mentioned point and beyond that point thus being established.

My invention will be further` described in connection with the accompanyingrdrawing which illustrates, diagrammatically and conventionally,

"of convection tubes in the flue l.

two of the several forms of apparatus adapted for the practice of my invention. In the accompanying drawing, Fig. 1 represents, in elevation and in section and with parts broken away, a heating furnace within which a heating conduit is arranged, Fig. 2 is a detail, in section, of a heating tube, shield and shield support, and Fig. 3 represents in elevation and in section and with parts broken away, another form of heating furnace within which a heating conduit is arranged.

Referring to Fig. 1, the heating conduit consists of a number of serially connected heating tubes `to which stock is supplied at 5 and fromwhich stock is discharged at 6. The flow of the stock, and its direction, is indicated in the drawing by the broken lines bearing arrows drawn through the heating tubes which appear in section. The stock rst passes through a bank of so-called convection tubes in the iiue 'I then through a bank of so-called radiant tubes on v-`theinner wall of the chamber 8, then through a divided bankV of radiant tubes on the outer wall lof the chamber 8 then through a bank of radiant tubes on the roof of the furnace structure and then, successively, through threebanks Shields 9 lare arranged over that part of the external surface of the `heating tubes in thebank on the 'inner wall and on the lower part of the bank `on the outer wall of the chamber 8 which more directly faces the locus of radiation. The cornbustiorof fuel, supplied through gas burners lil forexample, provides a source of heat largely available as radiant energy within the chamber 8'. V-Theproducts of combustion ow from the chamber 8 through the flue '1, in which the several banks of convection tubes are arranged, to a fan yII. Part of the waste heating gases discharged by the fan II may, with advantage, be recirculated through the chamber 8 with the Afresh products of combustion, and more particularlyfover the'walls of the chamber 8 on which the heating tubes are arranged, to assist in protection of the heating tubes. Such recirculated heating gases are returned through flue I2. Heating gases not recirculated through iiue I2 are discharged through flue I3. The stock to be cracked, supplied in liquid phase, is progressively heated as it flows through the first bank of tubes in the flue 'I and then through the bank of tubes on the inner wall of chamber 8 and the lower part of the bank of tubes on the outer wall of the chamber 8 until the change in phase from the lliquid state in which it is supplied to the state in which it is discharged is substantially completed, that is until phase change ceases. The operation is controlled, by regulation of the rate of supply of the stock and the rate of ring for example, vso that this point at which change in phase ceases is reached just before the stream of stock passes from the last shielded tube in the lower part of the bank on the outer wall of chamber 8 into the rst unshielded tube in this bank. It will be apparent that the lowermost bank of heating tubes in the flue 'I is effectively shielded from the locus of radiation in the chamber 8 by the inner wall of this chamber, so that the conduit in which the heating is effected is shielded from a point in advance of that at which substantial vaporization begins to the point at which the phase change substantially ceases.

Referring to Fig. 3, the heating conduit consists of a number of serially connected heating tubes to which stock is supplied at I 5 and from which stock is discharged at I6. The stock first chamber I8.

`of oneuseful form is illustrated in Fig. 2.

passes through a bank oi tubes on the roof of the furnace structure and then through a bank of tubes arrangedwithin the chamber I8 above the muiiles M. The combustion of fuel, supplied through gas burners 28 for example, is effected within the muiiles I Il. The products of combustion discharged from the muilles into duct II pass through the chamber I8 to the` stack flue 2 I. -Heat is transferred to the heating tubes from the heating gases passing through the chamber I8 and also, as radiant energy, through the upper walls of the muilles I 4, these walls being constructed of material, silicon carbide for example, adapted to permit the transmission of radiant heat. Shields I9 are arranged over that part of the external surface of the heating tubes in the bank on the roof of the furnace structure through which the stock first passes facing the cover of the duct I'I through which the products of combustion discharged from the muifles I4 enter the The stock to be cracked, supplied in liquid phase, is progressively heated as it flows through the bank of tubes on the roof of the furnace structure until the change in phase from the liquid state in which it is supplied to the state in which it is discharged is substantially completed,` that is until phase change ceases. The operationV is controlled, by regulation of the rate of supply of the stock and the rate of firing for example, so that this point at which change in phase ceases is reached just before the stream of stock passes from the last shielded tube in the bank on the roof of the furnace structure `into the first unshielded tube in this bank.

The precise form of shields and shield supports used is not vital to my invention. A detail In this form the shield 22 of' steel or an appropriate alloy, generally semicircular in section, is welded at intervals to U-bolts 23 by which it is clamped to the heating tube 24 to which it is applied. l A construction of this type, facilitating the placing and removing of the shields, is advantageous in that it permits adjustment of the apparatus to meet the requirements of varying operations.

In carrying out my invention, in the apparatus illustrated for example, the conduit in which the stock'is heated to a high cracking temperature is shielded from a point in advance of that at which substantial vaporization begins to the point at which the phase change substantially ceases to maintain a rate of heat transfer, through this part of the conduit, substantially lower than that maintained immediately beyond the point at which the phase change ceases, but this rate of heat transfer, through the shielded part of the heating conduit,I is nevertheless, sufficient to effect the phase change involved within the shielded Apart of the conduit. In referring to the point at which phase change ceases, it will be apparent that I mean to say that no substantial vaporization takes place beyond this point and not that vaporization is complete at this point. The metal' temperatures encountered along the conduit in which the stock is heated in carrying out my invention afford one measureof the advantage of my invention. If two operations, corresponding exceptin that in the second no differential between the heat transfer rates in advance of and immediately beyond the point at which phase change ceases is maintained, are compared by this standard, it will usually be found that much higher metal temperatures are encountered in advance of this point in the second thanin the first. VThat is to say, for the same capacity metal temperatures up to 420 F. at atmospheric pressure to a heating conduit substantially in liquid phase, heating the stock to a high cracking temperature in the heating conduit While flowing therethrough as a stream, discharging the heated stock from the heating conduit substantially in Vapor phase, maintaining a rate of heat transfer `per unit i of area of a portion ofsaid conduit extending beyond the point at Which the phase change substantially ceases such that overheating of said portion of said conduit will not occur but such that if this same rate of heat transfer were maintained throughout the entire conduit overheating would occur in a portion of the conduit in which takes placera part of the phase change, and maintaining a rate of heat transfer per unit of area of said conduit lower than the rate of heat transfer rst mentioned from a point in ad- Vance of that at which substantial vaporization begins to a point at which there has occurred an Vamount of Vaporization such that immediately beyond this point no further substantial overheating of the heating conduit will occur With the `rate of heat transfer rst mentioned.

2. In cracking petroleum stocks containing a` substantial proportion of gasoline for the production of gasoline fractions of improved anti-knock value, the improvement Which comprises supplying a stock of which more than 40% boils off up to 420 F. at atmospheric pressure to a heating conduit substantially in the liquid phase, heating the stock to a high cracking temperature `in the heating conduit while iiowing therethrough as a stream by heat transferred principally as radiant energy, discharging the heated stock from the heating conduit substantially in the vapor phase, maintaining a rate of heat transfer per unit of` area of a portion of said conduit extending beyond the point at which the` phase change substantially ceases such that overheating of said portion of said conduit will not occur in the absence of shields but such that overheating Would occur in a portion of said conduit in which takes place a part of the phase change if thissame rate of heat transfer were maintained throughout the entire conduit, and shielding said conduit from the source of said radiant energy from a point in advance of that at which substantial vaporization begins to the point at which there has occurred an amount of vaporization such that immediately beyond this point no further substantial overheating of the heating conduit will occur With the rate of heat transfer first mentioned.

. ORIN G. KAASA. 

